Fuel evaporative emission control systems have been in use on automotive vehicles for over 30 years. The gasoline fuel used in many internal combustion engines is quite volatile. The fuel typically consists of a hydrocarbon mixture ranging from high volatility butane (C-4) to lower volatility C-8 to C-10 hydrocarbons. When a vehicle is parked in a warm environment during the daytime heating (i.e., diurnal heating), the temperature in the fuel tank increases. The vapor pressure of the heated gasoline increases and fuel vapor will flow from any opening in the fuel tank. Normally, to prevent vapor loss into the atmosphere, the tank is vented through a conduit to a canister containing suitable fuel adsorbent material. High surface area activated carbon granules are widely used to temporarily adsorb the fuel vapor.
The fuel vapor enters the canister through a top inlet of the canister and into the carbon granule mass. The vapor diffuses downwardly under its own pressure and gravity into the volume of carbon granules where it is adsorbed in temporary storage. The total volume of adsorbent is specified so as to be suitable to retain a quantity of fuel vapor expected to evaporate from the fuel tank during normal or representative usage of the vehicle.
The canister is molded of a thermoplastic material and shaped so that ambient air can be drawn through the carbon granule bed during engine operation to purge adsorbed fuel from the surfaces of the carbon particles and carry the removed fuel into the air induction system of the vehicle. Typically, a partition is formed in the canister to lengthen the flow of vapor and air through the volume of carbon particles. Thus, the fuel vapor enters at one end, the vapor inlet, of the flow path and escapes at the opposite end, the vent outlet, if the quantity of fuel exceeds the adsorption capacity of the carbon volume. Air, induced to flow through the carbon under engine intake vacuum, enters the canister at the vapor vent end of the flow path. The air traverses the full length of the flow path and exits the canister with desorbed, i.e., purged, fuel through a purge outlet at the vapor inlet end of the carbon volume.
The described emission control system obviously works in a repeating cyclical mode. When the engine is not running, fuel vapor generated by diurnal heating, the previous return of hot fuel or the like, flows to the canister and is adsorbed up to the capacity of the adsorbent volume. The vehicle may remain idle for several days and fuel vapor will accumulate in the canister. The initial loading will be at the inlet end of the adsorbent volume, but the fuel gradually becomes distributed along the entire adsorbent bed pathway. When the vehicle engine is started and can accommodate a fuel-air mixture, a purge valve is opened and purge air is drawn through the adsorbent volume. Purging can continue as long as the engine is running and the air can cause the removal of a substantial portion of the stored fuel vapor.
Environmental regulators are proposing lower limits on the amount of fuel vapor that can escape the evaporative emission system during a prescribed test of the system in a closed space. For example, the California Air Resources Board (CARB) has proposed "zero" and "near zero" evaporative emission standards for automotive vehicles for year 2004. The proposed standards require that there cannot be any leaks in the vapor emission control system. Also, CARB onboard diagnostics regulations require that the evaporative emission control system diagnostics should be able to detect a 0.02 inch diameter leak in the system.
Recently, vehicles have incorporated diagnostic systems to detect problems or malfunctions in the operation of vehicle operating and emission control systems including the evaporative emission control system. However, in the case of the fuel vapor handling systems, there has been no practical on-board diagnostic procedure capable of detecting small leaks, of the order of 0.02 inch, in the evaporative emission control system of some vehicles, especially those equipped with vacuum compliant plastic fuel tanks. It is desirable to have such a diagnostic procedure that can be automatically performed by the vehicle computer control system and suitable complementary sensors to detect such small leaks and provide a notice of the leak to the vehicle operator.